Thermally pulsing asymptotic giant branch star models and globular cluster planetary nebulae – I. The model




Thermally pulsing asymptotic giant branch models of globular cluster stars are calculated using a synthetic model with the goal of reproducing the chemical composition, core masses and other observational parameters of the four known globular cluster planetary nebulae as well as roughly matching the overall cluster properties. The evolution of stars with an enhanced helium abundance (Y) and blue stragglers are modelled. New pre-thermally pulsing asymptotic giant branch mass losses for red giant branch and early asymptotic giant branch stars are calculated from the Padova stellar evolution models. The new mass losses are calculated to get the relative differences in mass losses due to enhanced helium abundances.

The global properties of the globular cluster planetary nebula are reproduced with these models. The metallicity, mass of the central star, overall metallicities, helium abundance and the nebular mass are matched to the observational values. Globular cluster planetary nebulae JaFu 1 and JaFu 2 are reproduced by assuming progenitor stars with masses near the typical main-sequence turn-offs of globular clusters and with enhanced helium abundances very similar to the enhancements inferred from fitting isochrones to globular cluster colour–magnitude diagrams. The globular cluster PN GJJC-1 can be roughly fitted by a progenitor star with very extreme helium enhancement (Y ≈ 0.40) near the turn-off producing a central star with the same mass as inferred by observations and a very low nebular mass. The abundances and core mass of planetary nebula Ps 1 and its central star (K648) are reproduced by a blue straggler model. However, it turned out to be impossible to reproduce its nebular mass, and it is concluded some kind of binary scenario may be needed to explain K648.